Depends on how the spell works. For example if you're asking a deity or spirit to watch over the target, just change the words and the focus from referencing yourself to referencing your sister. Some call for specific sigils to be drawn into the aura of the person protected or that they be in circle to be filled with some specific energy, and you would obviously need to instead perform those on your sister.

for luck you could draw the runic symbol of protection or luck in the earth then you could ask the person you are placing the spell on to lay down flat on the Ranice symbol on the ground. next you could chant a spell while you draw a circle around the person on the ground while visualizing the symbol being placed into the persons aura. once you feel the spell being in the persons aura. you spell is complete and the person may stand up and exit the engraved symbol in the earth.